1. Field of the Invention
The present invention relates generally to electronic circuits and, more specifically, to radio-frequency transceiver circuits, intended for very high frequencies (greater than 100 MHz).
2. Discussion of the Related Art
A problem which is particularly critical for high frequencies is that the system environment has a direct influence upon the impedance of the antenna. As a result, even for an antenna having good nominal characteristics in terms of ratio of the transmitted power to the reflected power (RL—Return Loss), this ratio may be disturbed by the environment, for example, when a user's hand comes close to the antenna. Now, high frequency ranges are widely used in mobile applications (cell phone, wireless connection of a portable computer, etc.) so that the effect of the human body (or another disturbing element) on the impedance of the antenna is not negligible.
Such modifications of the antenna's impedance have led, up to now, to interposing impedance matching circuits.
FIG. 1 is a block diagram illustrating a usual impedance matching solution. A transmit circuit 1 (SEND) is connected, via an integrated circuit 2, to a transceiver antenna 3. Circuit 2 comprises an adjustable impedance matching circuit 21 (MATCH). The impedance adjustment is performed by means of a first coupler 22 of distributed type, interposed between transmit circuit 1 and impedance matching circuit 21, and a second coupler 23, interposed between impedance matching element 21 and antenna 3. Coupler 22 provides, on an output terminal ISO of its secondary line, data relative to the power reflected by the antenna. Coupler 23 provides data relative to the power transmitted to the antenna to a detector 25 (DETECT) to reduce the insertion losses of circuit 21. The two detectors 24 and 25 provide the measured data to a control circuit 26 (CTRL) which adjusts the parameters of impedance matching circuit 21 according to a reference value (for example, 50 ohms) to reduce the insertion losses of circuit 21 and to improve the impedance matching at the level of transmit circuit 1. In the shown example, the case of a twin-wire connection between circuits 26 and 21, transmitting voltage data enabling matching of circuit 21, is considered. The matching circuit most often is an inductive and capacitive circuit (LC) having its capacitive elements settable by circuit 26.
When the antenna is disturbed by an external element, the modification of its input impedance is detected in the form of a variation of the transmitted and/or reflected power, which enables circuit 26 to modify the impedance of circuit 21 to maintain a matching supposed to be optimal between circuit 1 and antenna 3.
However, matching circuits generally have narrow operation bands, that is, they must be selected according to the frequency range for which the transceiver circuit is intended.
Further, the presence of a matching circuit adds losses in the transmission chain by the capacitive and inductive elements in series between the output of transmit circuit 1 and antenna 3.
Moreover, the power capacity is altered for the components forming circuit 21 when the mismatch is significant.